Revert "Just touch the timestamp for prism/srcs.mk when no baseruby"

This reverts commit 5a2cedd.

The CI is telling us to revert the diff:
https://github.com/ruby/ruby/actions/runs/17679836157/job/50251138515

Author: k0kubun

prism/srcs.mk

@@ -5,9 +5,6 @@ PRISM_CONFIG = $(PRISM_SRCDIR)/config.yml
 srcs uncommon.mk: prism/.srcs.mk.time
 
 prism/.srcs.mk.time:
-prism/$(HAVE_BASERUBY:no=.srcs.mk.time):
-	mkdir -p $(@D)
-	touch $@
 prism/$(HAVE_BASERUBY:yes=.srcs.mk.time): \
 		$(PRISM_SRCDIR)/templates/template.rb \
 		$(PRISM_SRCDIR)/srcs.mk.in